Sodium thiosulfate, Na₂S₂O₃, is used as a “fixer” in black-and-white photography. Suppose you have a bottle of sodium thiosulfate and want to determine its purity. The thiosulfate ion can be oxidized with I₂ according to the balanced, net ionic equation

If you use 40.21 mL of 0.246 M I₂ in a titration, what is the weight percent of Na₂S₂O₃ in a 3.232-g sample of impure material?

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1₂(aq) + 2 S₂O3² (aq) → 2 I¯(aq) + S₂0,² (aq)